A liquid crystal display device is provided with a backlight unit for irradiating a liquid crystal panel (LCD) with light from its rear side and illuminating the liquid crystal panel.
At present, a so-called bottom-lighting backlight unit is the mainstream for a large-sized liquid crystal television (refer to JP 07-36037 A, for example). The bottom-lighting backlight unit comprises a configuration wherein a plurality of cathode ray tubes serving as a light source is arranged on the rear surface of a liquid crystal panel, illuminating the liquid crystal panel with the inner area of the housing where the cathode ray tubes are arranged therein serving as a white reflection surface. Nevertheless, in this method, to ensure uniform light distribution, the thickness in the direction orthogonal to the liquid crystal panel needs to be about 30 mm, in principle.
In recent years, while there has been demand for a liquid crystal display device with reduced thickness, reduced power consumption, and increased size, the aforementioned bottom-lighting backlight unit has been limited in terms of reduced thickness due to the unevenness in brightness that occurs when the thickness thereof is reduced to 10 mm or less.
Among backlight units that allow reduction of thickness thereof is a backlight unit using a light guide plate in which light emitted by an illumination light source and entering the light guide plate is guided in given directions and emitted through a light exit plane that is different from the plane through which light entered.
There has been proposed a backlight unit of a type described above using a light guide plate formed by mixing scattering particles for diffusing light into a transparent resin (refer to JP 07-36037 A, JP 08-248233 A, JP 08-271739 A, and JP 11-153963 A, for example).
JP 07-36037 A, for example, discloses a light-diffusion light-guide light source device comprising a light-diffusion light guide member having at least one light entrance plane region and at least one light exit plane region and light source means for entering light through the light entrance plane region, the light-diffusion light guide member having a region that has a tendency to decrease in thickness with the increasing distance from the light entrance plane.
JP 08-248233 A discloses a planar light source device comprising a light-diffusion light guide member, a prism sheet provided on the side of the light-diffusion light guide member closer to a light exit plane, and a reflector provided on the rear side of the light-diffusion light guide member. JP 08-271739 A discloses a liquid crystal display comprising a light emission direction correcting element formed of sheet optical materials provided with a light entrance plane having a repeated undulate pattern of prism arrays and a light exit plane given a light diffusing property. JP 11-153963 A discloses a light source device comprising a light diffusion light guide member having a scattering power therein and light supply means for supplying light through an end plane of the light diffusion light guide member.
In the planar lighting devices provided with a light-diffusion light guide plate containing light scatterers mixed therein as disclosed in JP 07-36037 A, JP 08-248233 A, JP 08-271739 A and JP 11-153963 A, light emitted by the light source and entered the light-diffusion light guide member through the light entrance plane into receives a single or a multiple scattering effect at a given rate as the light propagates through the inside of the light-diffusion light guide member. Moreover, a significant proportion of light that reaches both end planes of the diffusion light guide member or a surface of the reflector receives a reflection effect and is returned back into the diffusion light guide member.
The above composite process produces a light beam that is emitted through the light exit plane highly efficiently with a directivity to travel obliquely forward as viewed from the light source. Briefly, light radiated by the light source is emitted from the light exit plane of the light diffusion light guide member.
Thus, the prior art literature mentioned above purportedly states that a light guide plate containing scattering particles mixed therein is capable of emitting uniform light with a high light emission efficiency.
Further, in JP A 11-153963, there have been disclosed a light guide plate in the form of a flat plate and a light guide plate composed of a portion shaped to have a region with a tendency to grow thinner with the increasing distance from the light entrance plane attached to the other portion, in addition to the light guide plate mentioned above that is shaped to have a region with a tendency to grow thinner with the increasing distance from the light entrance plane.    Patent Document 1: JP 07-36037 A    Patent Document 2: JP 08-248233 A    Patent Document 3: JP 08-271739 A    Patent Document 4: JP 11-153963 A